I use an SD card on my iLiad, and I have a card reader in the ExpressCard slot of my MacBook Pro.
I find it a lot more convenient to transfer items using the card reader in the ExpressCard slot (as it is always there and available, as if it was built in to begin with) than looking for an external reader and cable or using the clumsy iLiad adapter and cable evry time I want to transfer content
Anyways, whenever I am moving stuff onto my iLiad I make a backup of the SD card at teh same time.
Works very well for me.
I thought about making an Automator action or folder action to automate the process, but it just hasn't seemed worth it, since making a copy of the card manually doesn't require any real work (just an option-drag).
I suppose alternatively, you could use some backup software to copy only modifications, which would speed the backup process up...Carbon Copy Cloner is a donation-ware program that you could use.